225/40R18 vs 225/45R18 tire size
A 225/45R18 is 0.89 inches (22 mm) taller than a 225/40R18, and the same width. Fit them in place of 225/40R18 tires and the speedometer reads low: at 60 mph on it you’re doing 62.1 mph, 3.5% faster, and the vehicle sits 0.44 inches (11 mm) higher.
Sources checked
| Dimension | 225/40R18 | 225/45R18 | Change |
|---|---|---|---|
| Overall diameter | 25.09 in (637 mm) | 25.97 in (660 mm) | +0.88 in (+23 mm) |
| Section width | 8.86 in (225 mm) | 8.86 in (225 mm) | 0.00 in (0 mm) |
| Sidewall height | 3.54 in (90 mm) | 3.99 in (101 mm) | +0.45 in (+11 mm) |
| Aspect ratio | 40% | 45% | |
| Rim diameter | 18 in | 18 in | |
| Circumference | 78.8 in (2,002 mm) | 81.6 in (2,073 mm) | +2.8 in |
| Revolutions per mile | 804 | 777 | −27 |
| Revolutions per km | 500 | 483 | −17 |
Check another speed or size
Metric, LT and flotation
At 60 mph on the speedometer you’re doing
62.1mph
+3.5% · the speedometer reads low
- Height
- +0.89 in (+22 mm)
- Width
- 0.00 in (0 mm)
- Ground clearance
- +0.44 in (+11 mm)
- Each side, same wheel
- 0.00 in (0 mm)
225/40R18 and 225/45R18 side by side
| Dimension | 225/40R18 | 225/45R18 | Change |
|---|---|---|---|
| Overall diameter | 25.09 in (637 mm) | 25.97 in (660 mm) | +0.88 in (+23 mm) |
| Section width | 8.86 in (225 mm) | 8.86 in (225 mm) | 0.00 in (0 mm) |
| Sidewall height | 3.54 in (90 mm) | 3.99 in (101 mm) | +0.45 in (+11 mm) |
| Aspect ratio | 40% | 45% | |
| Rim diameter | 18 in | 18 in | |
| Circumference | 78.8 in | 81.6 in | +2.8 in |
| Revolutions per mile | 804 | 777 | −27 |
| Revolutions per km | 500 | 483 | −17 |
The working
- 225/40R18: sidewall = 225 mm × 40% = 90.0 mm = 3.54 in.
- Diameter = rim + 2 × sidewall = 18 + 2 × 3.54 = 25.09 in (637 mm).
- Circumference = π × 25.09 = 78.8 in; revolutions per mile = 63,360 ÷ 78.8 = 804.
- 225/45R18: sidewall = 225 mm × 45% = 101.3 mm = 3.99 in.
- Diameter = rim + 2 × sidewall = 18 + 2 × 3.99 = 25.97 in (660 mm).
- Circumference = π × 25.97 = 81.6 in; revolutions per mile = 63,360 ÷ 81.6 = 777.
- Speedometer. True speed = what it shows × new diameter ÷ old = 60 × 25.97 ÷ 25.09 = 62.1 mph.
- Ground clearance. Half the change in diameter: (25.97 − 25.09) ÷ 2 = +0.44 in.
Speedometer
A 225/45R18 turns 777 times a mile against the 225/40R18’s 804. With a speedometer set for 225/40R18, true speed is what it shows × 25.97 ÷ 25.09:
| Speedometer shows | True speed | Difference |
|---|---|---|
| 25 mph | 25.9 mph | +0.9 mph |
| 35 mph | 36.2 mph | +1.2 mph |
| 45 mph | 46.6 mph | +1.6 mph |
| 55 mph | 56.9 mph | +1.9 mph |
| 65 mph | 67.3 mph | +2.3 mph |
| 75 mph | 77.6 mph | +2.6 mph |
Clearance
- Ground clearance: +0.44 in (+11 mm), half the change in diameter.
- Top of the tread: +0.44 in (+11 mm), so that much less room under the fender.
- Each side, on the same wheel: no change.
A new wheel with a different width or offset moves the tire again. The sidewall goes from 3.54 to 3.99 inches.
What tire makers publish
Inflated on the maker’s measuring rim; the percentage is against the arithmetic.
| Tire | Size as printed | Diameter | Width | Revs per mile |
|---|---|---|---|---|
| Toyo Proxes Sport A/S | 225/40R18 | 25.1 in (+0.1%) | 9.1 in | 828 |
| Toyo Extensa A/S II | 225/45R18 | 25.9 in (−0.3%) | 8.9 in | 801 |
| Toyo Proxes Sport A/S | 225/45R18 | 25.9 in (−0.3%) | 8.9 in | 801 |
Each size on its own
- 225/40R18 tire size: 25.1 × 8.9 in
- 225/45R18 tire size: 26.0 × 8.9 in
Other comparisons
Questions
Can I fit 225/45R18 tires instead of 225/40R18?
They fit the same 18-inch wheels. A 225/45R18 is 0.89 inches (22 mm) taller than a 225/40R18, and the same width, so check three things first: that your wheels are within the 225/45R18’s approved rim width range, which tire makers list for each size; that it clears the fenders and suspension at full lock and full travel; and that its load index meets what your door-jamb placard asks for.
How far off is the speedometer with 225/45R18 tires?
With a speedometer set for 225/40R18, the speedometer reads low: at 60 mph on it you’re doing 62.1 mph, 3.5% faster. The odometer is off the same way: over 1,000 miles driven it shows 965.9.
Does 225/45R18 change the ground clearance?
By +0.44 inches (+11 mm), half the change in diameter, at the axles and at the lowest point under them. The width doesn’t change.
Sources
- Bridgestone Americas: Determining tire size: P-metric, Euro-metric, LT-metric, C-type and flotation sizes, and what each number means (checked October 28, 2026)
- Toyo Tires: Extensa A/S II sizes and specifications: inflated overall diameter and width, revolutions per mile (checked October 28, 2026)
- Toyo Tires: Proxes Sport A/S sizes and specifications: inflated overall diameter and width, revolutions per mile (checked October 28, 2026)
- Toyo Tires: Open Country A/T III sizes and specifications: P-metric, LT-metric and flotation sizes, with revolutions per mile (checked October 28, 2026)
- Nitto Tire: Trail Grappler M/T sizes and specs: inflated overall diameter and width, LT-metric and flotation sizes (checked October 28, 2026)
Spotted a number that doesn’t match your manual or a source? Tell us; corrections are logged on the sources page.